The invention relates in general to infrared image detection. More particularly, the invention relates to a polarization-sensitive infrared detection array using corrugated quantum well infrared photodetectors (C-QWIP) constructed to enable polarization-state detection of observed infrared radiation from a target object.
A quantum well infrared photodetector (QWIP) is a superlattice semiconductor device that functions to produce intersubband transitions within a conduction band when a ground state electron is promoted to an excited state upon absorbing an incoming photon having energy equal to the subband spacing. Once in an excited state, the electron freely moves within the QWIP to form electrical current under bias. As such, QWIPs are often used to detect infrared (IR) radiation, QWIP arrays have been used to detect IR images.
The physical construction of a conventional QWIP generally includes a stack of alternate semiconductor material layers sandwiched between two contact layers. The layers are grown on an IR transparent semiconductor substrate and cover an area that is relatively broad in comparison to the layer thickness. A typical semiconductor material system suitable for QWIP fabrication is InyGalxe2x88x92yAs/AlxGalxe2x88x92xAs. In a conventional QWIP device of the type described, intersubband transition can be initiated only if a component of the electric field vector of the incident IR radiation is normal to the broad surface areas of the semiconductor layers in the stack. Consequently, IR radiation that is incident normal to the semiconductor layers cannot be absorbed by the QWIP. Because only IR radiation having components propagated parallel to the semiconductor layers can be absorbed by the detector, attempts have been made to provide structures that can redirect the incident IR radiation closer to the desired parallel direction.
One known technique for redirecting IR radiation in a QWIP uses grating coupling techniques. The efficiency of a grating coupled with a QWIP is discussed in the following published article: Lundqvist et al., xe2x80x9cEfficiency of grating coupled AlGaAs/GaAs quantum well infrared detectors,xe2x80x9d Applied Physics Letters, vol. 63 (24), Dec. 13, 1993, pp 3361-3363.
Another technique for coupling light uses corrugated structures in the form of linear grooves that are chemically etched into active volume of the QWIP. Depending on alignment of these grooves with a certain crystallographic direction, grooves with different angled sidewalls can be formed. For example, grooves along a [011] direction have a V-shaped profile, while those along an orthogonal [011] direction have an inverted V-shaped profile. Likewise, the sidewalls of the grooves along either [001] and [010] are vertical. These corrugated structures redirect the normal incident light from the substrate side into more parallel propagation. For the V-grooves, the light is redirected at the angled sidewall through reflection. For the inverted V-grooves, it is by refraction. For the vertical sidewalls, it is by diffraction.
A detailed description of light coupling using a corrugated QWIP (C-QWIP) structure is taught in U.S. Pat. No. 5,485,015 by my previous invention entitled xe2x80x9cQuantum Grid Infrared Photodetectors,xe2x80x9d and an article by me, entitled xe2x80x9cThe Physics of Quantum Well Infrared Photodetectors,xe2x80x9d (World Scientific, River Edge, N.J., 1997) pp. 189-199. In these teachings, detection is primarily concerned with intensity measurements of the radiation. However, the use of grating coupling or corrugation coupling to detect polarization characteristics of incident infrared radiation is not suggested or taught by these teachings. The additional polarization information inherent in the observed infrared radiation has important details as to the surface properties of an observed target object. The detection and characterization of the object can be based on the difference in the state of polarization of the radiation from that object relative to a background scene. Such a problem is resolved by the present invention.
Corrugated quantum well infrared photo-detector (C-QWIP) arrays are sensitive and have high-resolution thermal imaging capabilities. These arrays are typically made out of low cost type III-V semiconductor materials and are easily manufactured in large quantities. They are suitable for a wide range of conventional applications as well as offering new imaging capabilities. C-QWIP arrays can be produced by a small number of conventional processing steps. They can be applied to most conventional applications, such as night vision, missile tracking and guidance, air and land traffic safety, environmental protection, natural resources exploration, meteorology, medical imaging, infrared astronomy, quality control, and robotic vision.
Areas where polarization-sensitive detection technology has been studied and investigated have used conventional technologies. Most of this study has been related to landmine detection. Polarization-sensitive cameras have been discussed in the following articles that include: (1) Blair A. et al.,xe2x80x9cPassive IR polarization sensors: a new technology for mine detection,xe2x80x9d SPIE Vol. 3392 (April 1998), pp 96-103; (2) Larive M. et al. xe2x80x9cLaid and flush-buried mines detection using 8-12 micrometer polarimetric imager,xe2x80x9d SPIE Vol. 3392 (April 1998), pp.115-120. In these studies, the IR cameras are not intrinsically polarization sensitive (i.e. these cameras differentiate different levels of intensity, not polarization-state of the observed radiation). For polarization detection, both of these teachings use a set of optical elements, such as filters, polarizers and quarter-wave plates, placed in front of the camera. These optical elements sort out the polarization components contained in the radiation sequentially before this radiation is incident upon the camera. In such a polarization detection scheme, a series of infrared images have to be taken as the optical elements sort out the polarization at different directions. These earlier polarization detection systems taught by these studies have limitations that include: (1) The optical elements in such systems have to be displaced or rotated either manually or by automated mechanisms in a time-delayed manner since they are intended to observe an image with still objects. Any objects moving in an observed scene corrupts polarization characteristics of an object under observation. (2) Such a detection camera system is more expensive to implement due to additional hardware requirements of optical elements and automated control mechanisms wherein these additional hardware components also add weight and volume to such systems. (3) Since these optical elements of these systems are wavelength specific, the camera can only image the scene at a very narrow range of wavelengths. (4) The optical elements in these systems attenuate the incoming radiation. The last two factors reduce the sensitivity of the camera.
In view of these limitations, there is need for a simple and inexpensive polarization-sensitive infrared detector array for use in cameras and similar instruments, which the present invention provides.
The invention relates to polarization-sensitive infrared detector arrays for use in cameras and other instruments, which are comprised of a collection of C-QWIP detector units. The array is preferably in a two-dimensional array that can detect polarization contrast of an observed target object in a scene. Each detector unit is formed by a group of C-QWIP detectors having different groove orientations. Each detector unit is comprised of at least two C-QWIP detector elements with their respective corrugations oriented in orthogonal directions. Infrared detection by these detector units is primarily by polarization contrast, compared to intensity contrast, which is well known in the art. Also, by measuring polarization of reflected light from an observed target material, nature of this reflecting material can also be identified.
In a first embodiment of the invention, each detector unit comprises four C-QWIP elements. These detector units are further divided into two pairs. Each pair of C-QWIPs has groove orientation orthogonal to each other. The orthogonal directions form the principal axes for characterizing polarization-state of the radiation. The angle between these two pairs of axes is arbitrary. Also, in this embodiment of the invention, at least four C-QWIP detector elements are required in each unit for determination of a polarization state of incident radiation on in the detector unit. At least one detector unit is required to form an array, which can comprise many detector units in a unitary structure. A signal from each detector unit varies according to whether there is intensity variation or polarization variation. The polarization selectivity is intrinsic to each of the detector units. Each C-QWIP in a detecting unit is sensitive to one polarization, that is the polarization with the electric-field vector being perpendicular to the grooves.
A second embodiment of the invention comprises a detector unit having fewer than four C-QWIPs in individual detector units. Advantages of reducing the number of detector elements per unit include the ability to have more units built into an imaging array having a fixed area, thus increasing the resolution of the array. However, in this second embodiment, such detector units cannot completely resolve all the polarization properties of the radiation. Signal information from such detector units is partly dependent on the intensity and partly dependent on the polarization properties of the incident radiation thereupon. Such information is useful in applications requiring detection of objects without need for knowing all the polarization properties. This second embodiment is applicable in cameras or navigational instruments where radiation from an observed target object in a scene is highly polarized.
Applications of the invention using either embodiment of the polarization-sensitive detector arrays using C-QWIPs elements include infrared (IR) cameras and other instruments that can be used for landmine detection, missile interception, fire fighting, airplane deicing, navigational aid instruments, 3-D cameras and material surface inspection for quality control.
Accordingly, several advantages of the present invention include:
(a) Providing a polarization-sensitive infrared detector array using integrated C-QWIP detector elements to obviate the need for polarizing optical elements in IR detecting instruments.
(b) Providing a polarization-sensitive infrared detector array that acquires polarization-state information simultaneously.
(c) Providing a polarization-sensitive infrared detector array that is inexpensive to produce, which can be incorporated into cameras that are physically lighter and smaller compared to presently used IR based cameras with similar detection capabilities.
(d) Providing a polarization-sensitive infrared detector array comprised of corrugated quantum well infrared photodetectors (C-QWIP) units having at least two elements, wherein each unit is dependent of polarization.
The exact nature of this invention and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings wherein like reference letters and numerals correspond to like parts throughout the several views.